1. Field of the Invention
The present invention relates to a color cathode ray tube of shadow mask type, and particularly to a color cathode ray tube of shadow mask type having a so-called flat face such that the shape of a face plate panel is substantially planar.
2. Prior Art
A cathode ray tube (hereinafter referred to as a color cathode ray tube) used for color image display is constituted by a panel portion which is an image screen, a neck portion for accommodating an electron gun, and a funnel portion coupling the panel portion (face plate panel) to the neck portion, and the funnel portion is equipped with a deflector which deflects the electron beam emitted from the electron gun to scan the fluorescent screen formed on the inner surface of the panel.
Against the inner surface of the face plate panel is installed a shadow mask which is a color selection electrode having a curvature that is similar to the curvature of the inner surface, in order to define the positions at which a plurality of electron beams emitted from the electron gun impinge upon the fluorescent screen.
The electron gun accommodated in the neck portion has various electrodes such as a cathode electrode, a control electrode, a converging electrode and an accelerating electrode, and modulates the electron beam from the cathode electrode with a signal applied to the control electrode, imparts a desired sectional shape and energy thereto through the converging electrode and accelerating electrode, and causes it to impinge upon the fluorescent screen. The electron beam is deflected in the horizontal direction and in the vertical direction by the deflector provided in the funnel portion in the way from the electron gun to the fluorescent screen, and forms an image on the fluorescent screen (see, for example, Japanese Patent Laid-Open No. 215640/1984).
There has been a tendency for recent color cathode ray tubes to have a so-called flat face, namely the shape of the face plate panel on which an image is formed is substantially planar.
Flatness of the face plate panel of color cathode ray tubes is greatly affected by the curvature of the peripheral portion (periphery of the face plate panel). Therefore, the flatness of the face plate panel increases with a decrease in the curvature of the peripheral portion. However, the shadow mask has a flat shape, too, to meet the curvature of the face plate panel resulting in an increase in the landing error (doming phenomenon) due to the thermal expansion of the shadow mask, causing the poor image reproduction.
A color cathode ray tube which has realized both the flattening of the face plate panel and improvements in the doming phenomenon is disclosed in Japanese Patent Laid-Open No. 232247/1988.
According to the invention disclosed in the above publication, the shapes of the inner and outer surfaces of the face plate panel are expressed by three-dimensional spatial equations in which the X-axis is the long axis (horizontal axis), the Y-axis the short axis (vertical axis) and the Z-axis the axis of the tube. That is, curved surface shapes Z.sub.X, Z.sub.Y on the X-axis and Y-axis are approximated by Z.sub.X =A.sub.1 X.sup.2 +A.sub.2 X.sup.4 and Z.sub.Y =A.sub.3 Y.sup.2 +A.sub.4 Y.sup.4, and curved surface shapes P.sub.X, P.sub.Y at a point (X=X.sub.1, Y=Y.sub.1) of an effective screen boundary portion which is the boundary between the effective screen portion and the surrounding peripheral portion are approximated by P.sub.X =A.sub.1 X.sub.1.sup.2 /(A.sub.1 X.sub.1.sup.2 +A.sub.2 X.sub.1.sup.4) and P.sub.Y =A.sub.3 Y.sub.1.sup.2 /(A.sub.3 Y.sub.1.sup.2 +A.sub.4 Y.sub.1.sup.4), wherein the above constants A.sub.1, A.sub.2, A.sub.3 and A.sub.4 are set to satisfy 0.3.ltoreq.P.sub.X (X=X.sub.1).ltoreq.0.6 and 0.95.ltoreq.P.sub.Y (Y=Y.sub.1).ltoreq.1.0. Furthermore, the effective screen boundary portions on the outer surface of the face plate panel on the short side and on the long side are formed to have arcuate shapes with nearly an equal curvature and that a relationship 1.5(42.5 V+45.0).ltoreq.R.ltoreq.2.0(42.5 V+45.0) is satisfied when the radius of curvature R(mm) thereof has an effective diagonal diameter of V (inches), to thereby reduce the doming of the shadow mask and to improve the strength, image reflected on the surface and flatness of the face plate.
In conventional color cathode ray tubes having the above mentioned curved surface shapes, however, when it is attempted to further flatten the face plate panel causing the radius of curvature of the peripheral portion to lie outside the above mentioned R (mm) range, the image reflected on the outer surface is greatly distorted and, in particular, local distortion increases greatly. Moreover, a landing error increases due to doming of the shadow mask, seriously impairing the quality of the image that is displayed.